Form submit event is not capturing OnSelectedIndexChanged?

Form submit event is not capturing OnSelectedIndexChanged?

I have a button and a dropdownlst

And I have this script :
$("form").submit(function (e)
{
$('#divOverlay').show();
});
My Goal :
When a form submits , I need to show a "loading div" :
Question :
When I press the button it does show me the div (the gray div which blinks
for a second):

But when I change index on the dropdownlist :
<asp:DropDownList runat="server"
OnSelectedIndexChanged="OnSelectedIndexChanged" AutoPostBack="True">
<asp:ListItem Value="a"> a</asp:ListItem>
<asp:ListItem Value="b">b</asp:ListItem>
</asp:DropDownList>
It does submit bUt I dont see the div :

Why does the $("form").submit(function (e) does not capture this postback
which occurs after selected index change ? ?
NB :
Fiddler shows both events (pressing the button && changing index) as a
POST command